This invention relates to measurement devices, and more particularly to an electro-optic device for measuring the fullness level of liquid in a container.
Continuous level sensing is used for a vast number of applications, perhaps the most familiar being for tanks containing liquids, such as a fuel tank. Unlike, a limit levelmeter (also known as a switch levelmeter), a continuous levelmeter must provide a continuous range of measurements from empty to full.
There are many different types of level meters, each type having a different principle of operation. Some of the more common types are float levelmeters, capacitive levelmeters, photoelectric levelmeters, and ultrasonic levelmeters.
Some levelmeters, notably ultrasonic levelmeters, have used signal frequency to provide level information. For example, one type of ultrasonic levelmeter uses an emitter to direct ultrasonic waves into a cavity above the liquid. The resulting waves resonate at the cavity""s resonant frequency and at harmonics of that frequency. At a different liquid level, the resonant frequency is different. Thus, measurement of the oscillation frequency provides a measure of the liquid level.
One aspect of the invention is an opto-electric levelmeter for measuring the level of liquid in a container, comprising: a sensor that has at least a light emitting diode and a phototransistor. The sensor is operable to provide a sensor output signal whose frequency is related to the distance of the sensor from the surface of the liquid. More specifically, this distance is an xe2x80x9coptical linkxe2x80x9d and is connected into the feedback loop of an oscillation circuit within the sensor. The sensor delivers this output signal to a remote monitor that has level detection circuitry for converting the sensor output signal into a signal representing the level of liquid in the container. The monitor has an adjustment circuit that permits adjustment of both the high and the low level of the level detection circuitry.
One advantage of the levelmeter is the low cost of its sensor. The sensor may be used with a remote monitor and is easy to install. It is not invasive as is a float type levelmeter, and it does not require a wide orifice as does an ultrasonic type levelmeter.
Because the sensor output is a frequency not an amplitude, the signal may be transmitted long distances. Resistance variations on a transmission line are less likely to affect the signal readings. The sinusoidal oscillations generate low electromagnetic interference.
The levelmeter has a built in signal transmitter, in that the same wire that delivers electrical energy to the sensor may also be used to transmit the sensor output signal to the remote monitor. Two independent settings, one for low level and one for a full level, permit the use of one sensor configuration for different container sizes, where the level of fullness is provided to the user as a percentage. The device may also be quickly and easily calibrated.